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  <title>Document</title>
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  <script>
    function sayHi(n=0,m=0){
          return n+m
          // return  返回的关键字   后面不能再写代码
    }
        let he = sayHi(4, 5)  //   sayhi 此时可以拿到返回值，然后再被赋值到he
        console.log(he);
  </script>
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